Method and apparatus for coating electroluminescent phosphors

ABSTRACT

A process for preparing particles of zinc sulfide-based electroluminescent phosphor having a moisture resistant coating thereon which comprises the steps of selecting a reaction vessel having a given height and a porous disc at the bottom thereof; charging the reaction vessel with phosphor particles and fluidizing the particles by introducing an inert gas into the vessel through the porous disc; heating the reaction vessel to a reaction temperature; introducing a coating precursor into the reaction vessel at a position adjacent the bottom of the vessel but above the disc; introducing a co-reactant into the reaction vessel at a position substantially mid-way of the given height; and maintaining the inert gas flow, the precursor flow and the co-reactant flow for a time sufficient for a reaction to occur and coat the phosphor with the moisture resistant coating. Apparatus for carrying out the process is also disclosed.

This application claims priority from Provisional Patent Application No. 60/256,194, filed Dec. 18, 2000.

TECHNICAL FIELD

This invention relates to zinc sulfide-based electroluminescent phosphors and more particularly to a method and apparatus for coating such phosphors with a moisture inhibiting coating.

Still more particularly, it relates to a method and apparatus for making such phosphors in commercial quantities.

BACKGROUND ART

It is known from U.S. Pat. No. 6,064,150 that a coating of aluminum nitride or aluminum nitride amine on an electroluminescent phosphor particle allows the manufacture of electroluminescent light sources having good life characteristics. It is also known to coat such phosphors with a metal oxide hydroxide.

It would be an advance in the art if such coatings could be applied to large batches of phosphor materials.

DISCLOSURE OF INVENTION

It is, therefore, an object of this invention to obviate the disadvantages of the prior art.

It is another object of the invention to enhance the manufacture of commercial quantities of coated electroluminescent phosphors.

These objects are accomplished, in one aspect of the invention, by a process for preparing particles of zinc sulfide-based electroluminescent phosphor having a moisture resistant coating thereon which comprises the steps of selecting a reaction vessel having a given height and a porous disc at the bottom thereof; charging the reaction vessel with phosphor particles and fluidizing the particles by introducing an inert gas into the vessel through the porous disc; heating the reaction vessel to a reaction temperature; introducing a coating precursor into the reaction vessel at a position adjacent the bottom of the vessel but above the disc; introducing a co-reactant into the reaction vessel at a position substantially mid-way of the given height; and maintaining the inert gas flow, the precursor flow and the co-reactant flow for a time sufficient for a reaction to occur and coat the phosphor with the moisture resistant coating.

The apparatus for manufacturing commercial quantities of zinc sulfide-based electroluminescent phosphor having a moisture resistant coating thereon, comprises a reaction vessel having a given height and a porous disc at the bottom thereof; a supply of phosphor particles within the vessel; a first supply of an inert gas for fluidizing the particles, the first supply of inert gas entering the vessel through the porous disc; a heater surrounding the reaction vessel for heating the reaction vessel to a reaction temperature; a supply of a coating precursor; first means for conducting the coating precursor from the supply to the reaction vessel, the first means entering the reaction vessel at the bottom of the vessel at a position above the porous disc; a supply of a co-reactant; and a second means for conducting the co-reactant from the supply to the reaction vessel, the second means entering the reaction vessel at a position substantially mid-way of the given height.

This method and apparatus allows for the commercial production of sizeable quantities of coated electroluminescent phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE is a diagrammatic illustration of the apparatus of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in conjunction with the above-described drawings.

Referring now to the drawings with greater particularity, there is shown in the FIGURE a reaction vessel 10 a that is preferably formed of stainless steel having a diameter of greater than 6 inches, a height of about 6 feet and a capacity of greater than 50 kilograms. In a preferred embodiment, the vessel 10 a has a diameter between 10 and 12 inches, a height of 6 feet and a capacity of greater than 150 kilograms. In actuality, capacities of more than 300 kilograms are possible. The vessel 10 a is surrounded by a suitable heater 30 a that is capable of bringing the reaction vessel 10 a to coating temperature between 150 and 225° C. The coating precursor, which can be hexakis(dimethylamido)dialuminum, is fed from a supply 20 a after entrainment with nitrogen from a supply 22 a thereof through tube 32 into the vessel 10 a. The entrance of tube 32 into vessel 10 a is at a point about 1.0 cm above a porous disc 12 a. The tube 32 has a narrow tip within the vessel 10 a, the tip being located in the center of the reaction vessel.

The co-reactant, which can be diluted anhydrous ammonia, is fed from a supply 24 a into the side of the reaction vessel 10 a, via a tube 33, at a point substantially mid-way of the height. Tube 33 has a narrow opening slit that is located in the center of vessel 10 a. The co-reactant can pass through a Unit mass controller 26 a prior to entering the vessel 10 a. Dilution can be accomplished by adding purified nitrogen.

A supply of inert gas 18 a such as nitrogen is provided for fluidizing the phosphor particles 16. The inert gas is fed from the bottom of the reaction vessel 10 a, through the porous disc 12 a, thus ensuring a good fluidization of the particles.

This process and apparatus solve the problems of the prior art technique wherein the precursors, such as the above-cited hexakis(dimethylamido)dialuminum were charged into the reaction vessel through the porous disc 12 a. It has been found that the relatively extreme reactivity of the precursor caused a nitride-plating reaction to occur within the pores of the disc 12 a, soon plugging the holes in the disc and stopping the desired reaction. This occurs even more rapidly because of the elevated temperatures of the reaction vessel, which are between 150 and 225° C.

By charging the precursor into the reaction vessel above the porous disc the plating problem is eliminated. Also, by feeding the nitride precursor from the side instead of from the top of the reaction vessel, as did the prior art, the precursor is exposed for a shorter time inside the hot vessel. Additionally, this reaction vessel design can be utilized with other coating processes, such as the reaction of trimethylaluminum with oxygen and ozone. Feeding an oxygen-ozone mixture from the side of the vessel instead of from the top extends the life of the very reactive ozone within the vessel allowing greater coating potential.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modification can be made herein without departing from the scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A process for preparing particles of zinc sulfide-based electroluminescent phosphor having a moisture resistant coating thereon comprising the steps of: selecting a reaction vessel having a given height and a porous disc at the bottom thereof; charging said reaction vessel with phosphor particles and fluidizing said particles by introducing an inert gas into said vessel through said porous disc; heating said reaction vessel to a reaction temperature; introducing a coating precursor into said reaction vessel at a position adjacent said bottom of said vessel but above said disc; introducing a co-reactant into said reaction vessel at a position substantially mid-way of said given height; and maintaining said inert gas flow, said precursor flow and said co-reactant flow for a time sufficient for a reaction to occur and coat said phosphor with said moisture resistant coating.
 2. The method of claim 1 wherein said moisture resistant coating is aluminum nitride amine.
 3. The method of claim 1 wherein said coating precursor is hexakis(dimethylamido)dialuminum.
 4. The method of claim 3 wherein said co-reactant is anhydrous ammonia.
 5. The method of claim 4 wherein said reaction temperature is about 150 to 225° C. 